Exodus 7:19

Streams.. rivers] the various canals and branches of the Nile.

20, 21. At the annual rising of the Nile its waters frequently turn a dull red colour owing to the presence of mud, vegetable débris, and minute animalcules. This plague, therefore, like the following, may have been an aggravation of a natural phenomenon. It is to be observed, however, that whereas the natural discoloration of the water has no pernicious effect on the fish of the Nile, these all died under the plague.